Pushbuton selection



March 29, 1966 E. HERTER PUsHuTToN sELncTIoN Filed Feb. 27, 1963 2 Sheets-Sheet 1 wat INVENT OR EBEPHARD #M722 /HZWZ// an Ud ATTORNEY United States Patent O 3,243,516 PUSHBUTTON SELECTION Eberhard Herter, Stuttgart, Germany, assigner to Interna- The invention relates to D.C.signalling methods, and more particularly to pushbutton selection in telephone systems, wherein the code signals are evaluated using two interrogating steps of different current direction.

There are various asymmetrical D.C. signalling methods known wherein the code signal formed asymmetrically is evaluated in two interrogating steps of different current direction. In one of the interrogating steps, however, the current direction is the same in both conductors. The following conditions serve as code elements on the transmitting end: either of the conductors is grounded, both conductors are grounded, or no conductor is grounded. The code elements in both current directions are separated by rectiiers. Thus, by using asymmetrical methods 16 combinations are possible. However, all the combinations cannot be used exclusively for signalling by pushbutton selection because the conditions subscriber has lifted the handset and subscriber has restored the handset must also be communicated by the direct current. A part of the combinations therefore cannot be used as dialling signals.

A problem in the aysmmetrical D.C. signalling methods is the signal criterion, which is a signal for indicating on the receiving end when selection signals are being emitted. Loop interruption is commonly used as the criterion. This loop interruption occurs when the grounded terminal of a battery in a circuit is switched over to the two interrogating voltages of same polarity. The evaluation of the signal criterion practically is a third interrogation step and, consequently, results in a considerable reduction of the signalling time. Any change in the voltage drop at the supply resistors regardless of whether one, both or no conductor is grounded is indicative of signalling. These known solutions, however, have another disadvantage. Provisions must be made to insure that all contacts of a dialling key close simultaneously. If the contacts close successively, grounding of one conductor causes a change of the voltage drop across the supply resistors. Thus, a false signal criterion is received and the facility is switched over to the interrogating voltages of the lirst interrogating direction. lf the pushbutton is pressed down slowly, the loop via the subset may be closed during this premature interrogation resulting in a wrong condition.

In order to avoid these faults a particularly designed set of keys is required in which the operating sequence of the contacts is subject to certain prerequisites. Such an arrangement is not only complicated and expensive, but also not safe in operation. Another possibility of preventing these faults is to derive the signalling criterion form the interruption of the loop. This, however, requires the ground to be applied via a resistor for the signalling so that it is possible to discriminate between signals by the different voltage drops at the supply resistors. Since induced noise voltages impair the asymmetrical signalling method, the admissible interfering voltage is reduced by this arrangement because in asymmetrical signalling circuits a subdivision of the current range is also made.

3,243,516 Patented Mar. 29, 1966 It is an object of this invention to provide a new inexpensive and noise immune asymmetrical D.C. signalling system.

It is another object of this invention to provide a signalling system in which ground is directly applied to the conductors without using particularly designed key sets.

It is still another object of this invention to reduce the admissible interfering voltage.A

Still a further object is to provide an asymmetrical D.C. signalling system that is not affected by wide variations in supply voltage.

The asymmetrical D.C.signalling method, according to the invention is characterized in this, (l) the polaritiesof the interrogating voltages are different at each interrogating step, (2) the interrogating voltages for the iirst current direction are applied even if the signalling device is in a non-operated, rest position, (3) for the iirst interrogating direction on the transmititng end only those code elements will be used (one of two or no conductor grounded), after loop interruption on the transmitting end which effect a change in potential on one or both conductors of the line, and (4) the signal criterion and the signal itself for the first interrogating direction is represented simultaneously by the potential changes up to the value of an interrogating voltage.

lt is a feature of this method that recognition of the signal criterion and evaluation of the first interrogating step coincide. By using a battery provided with a center grounding for the two differently pole interrogating voltages, it is possible for the telephone set to be grounded with both conductors. Application of ground through the signal contacts therefore does not cause any voltage change in the conductors of the line. The code elements are evaluated via the potentials at the conductors. The need to respond to the pressing of the selecting keys reduces the available signals of the first interrogating step, since grounding applied to both conductors does not cause any potential change on the conductors during the loop interruption and can therefore not be used as a code element. For the second interrogating step both interrogating voltage will be changed in their polarity. However, all four code elements can be used resulting in 3 x 4=12 possible combinations.

After the second interrogating step the voltages of the first interrogating step are re-applied, so that the end of signalling can be supervised. If more combinations are desired, closing of the loop for the second interrogating direction can be used, according to the invention, as an additional code element. Detection of this element from the code element both conductors grounded Will be initiated on the receiving end so that one interrogating voltage will be switched off. This detection in a third interrogating step is required because the potential conditions on both conductors are practically the same in both code elements mentioned above. If, however, one voltage will be switched olf, there are other potential conditions which mark either one or the other of the two code elements. After the third interrogation the voltages are restored to the values of the voltages of the first interrogating step.

A rectifier provided for the asymmetrical signalling, is also used -to form the additional code element. In constructing the key set it is required that the loop is interrupted only after closing of the respective signal contacts.

The signalling method according to the invention will be explained in detail with reference to the iigures of the accompanying drawings, in which:

FIG. 1 shows a circuit embodying the principle of the invention signalling method,

FIG. 2 shows the signalling code, and

FIG. 3 shows the voltage conditions for the evaluation device.

FIG. 1 shows the circuitry of the telephone set 1 connected with the exchange oice ready for speaking via break contact 2 and the hookswitch 3. The diodes D1 to D4 and lthe pushbutton contacts k1 to k7 `are provided to form the code signals. The signals are evaluated by the measuring devices M1 and M2 which evaluate the voltages Va between conductor a and ground and Vb between conductor b and ground according to amplitude and polarity. The telephone set is supplied the two interrogating voltages V1 and V2 in such a Way that the conductor a initially receives negative, and the conductor b receives positive potential. This connection of the voltage sources changes for the second interrogating step in which the voltage conditions on the conductors are just reversed via the contacts K8 and K9 actuated by the reversing relay. As the break contact K10 indicates, switching-ofic of the interrogating voltage (V1.) is possible for a third interrogation.

FIG. 2 shows the combinations possible with the signalling method according to the invention. As already described the condition both conductors grounded in the first interrogating direction cannot be used, because in 4the following loop interruption the voltages Va and Vb remain unchanged. In both cases, Va and Vb are practically of the same value. Only a small change in voltage occurs because the resistor in the subset cannot be made arbitrarily small. If after the loop interruption, the conductor is not grounded, the voltage in the conductors a, b reaches the value of the interrogating voltage V1 or V2 respectively. If the conductor is grounded, ka voltage Va or Vb respectively is obtained which rises from zero to a value dened by `the maximum line resistance and the supply resistors R. If, for example, the maximum line resistance equals the supply resistor, Va or Vb respectively receive voltage values up to half the voltage V1 or V2 respectively. These voltages Va and Vb are always of different polarity in the rst two interrogating steps.

If the subscriber has closed the loop, the no-signal position is given by the voltages Va `and Vb shown in FIG. 3. Va is negative and smaller than half the voltage V1. Vb is positive and smaller .than half the voltage V2 and always stays constant. At any arbitrary signal combination at least one of these voltages Va and Vb will change to the value of the interrogating voltage V1 or V2 respectively after contact 2 4has opened. Contact 2 opens responsive to the operation of any of the keys K1-K7. In the combinations 1 to 5 marked by the grounded conductor a for the rst current direction Vb will have the value of V2. In the combinations 6 to 10 marked by the grounded conductor b the voltage Va rises to V1. In case that both conductors are not grounded for the first interrogating step, applicable for the combinations 11 to 15, the two voltages Va and Vb change to V1 or V2 respectively. On the receiving end the -criterion for signalling may be clearly derived. The voltage conditions on the conductors a and b prevailing when receiving this signal criterion, are the first part of `the D.C.code signal. After evaluation of :these voltages practically simultaneously with the evaluation of the signal criterion the circuit is switched over to the second interrogating-current direction. The negative interrogating voltage V1 is applied to conductor b and the positive interrogating voltage V2 to the conductor a. The code elements of the second interrogating step are either one wire grounded or .the other Wire grounded, both wires grounded or no wire grounded. Consequently, the voltage conditions on the conductors change. The polarity of both voltages Va and Vb change with different code elements. The .amplitude of the voltages also depends on the code element. If conductor a is grounded, Vb takes the value of V1. If conductor b is grounded, the voltage Va rises to voltage V2, and if both conductors are grounded, both voltages remain small. Since, however, `the code element conductors connected is used too as indicated by the key k7 of FIG. l these voltage conditions must be detected Whether they come from one or from the other code element. This necessitates a third interrogation. If at -the second interrogation the above voltage conditions occur, (Va and Vb both have low values and are of opposite polarity) the voltage source V1 will be separated from the conductor b via contact K10 responsive to the operation of the loop relay. If both conductors are grounded, only one voltage Va occurs with the same polarity as in the second interrogation. If, however, the code element loop closed -is given voltage Va and Vb will become positive with `the amount of voltage V2. As this example shows it is sucient to evaluate in the third interrogating step only one of the two voltages Vcz or Vb in order to receive `the information on the code element at hand. If during the second interrogating step one of the voltages Va or Vb has reached the value of an interrogating voltage V1 or V2 respectively, a third interrogation is not required. FIG. 3 shows the voltage conditions of all combinations. The shaded voltage ranges depend on the line resistances. For evaluation it suffices to provide switching means responsive to the entire interrogating voltage but not responsive to the partial voltage due to the maximum line resistance. To distinguish between the combinations, iive switching means are suflicient, four of which are used in connection with the condition of the yiirst and second interrogating step (relay 1 4). The ifth switching means (Loop Relay) only participates in t-he third interrogating step to separate the signals 3 .and 4, 8 and 9 as Well as 13 and 1d..

The principle of the invention explained with the aid of a pushbutton-selection method is not limited to that application only. In any system wherein many signals are required to be eciently 'transmitted to a center with a minimum of expenditure, the inventive method can be applied. It is `a great advantage of this invention that `the signalling is still coherent even with larger interfering voltages than with the methods hitherto known. It is unimportant which combinations may be used to indicate the ycondition subscriber has restored handset or subscriber has lifted handset. To represent these signals contacts of the cradle switch may be used.

While the principles of the invention have been described above in connection With specific apparatus, it is to bc clearly understood -that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

1. A signalling arrangement for a pushbutton telephone dialling system comprising central oice equipment serving a plurality of two conductor lines, each having a subscriber station thereon, voltage supply means in said central oice having a positive voltage terminal for supplying a positive voltage with respect to ground and a negative voltage terminal for supplying negative vol-tage with respect to ground, interrogating means in said central ofce normally connecting said supply means in a rst interrogating position with said negative terminal Connected to the rst of said two conductors and said positive terminal connected to the the second of said ltwo conductors, a plurality of digit signalling means in each of said stations selectively operable to mark said conductors in desired permutations and combinations, said permutations .and combination comprising individually grounding the first conductor, individually grounding the second conductor, grounding both conductors simultaneously, grounding neither of the conductors and connecting both conductors together, evaculating means in said central oice mounted between each of the conductors and vground and operated responsive to said markings, and

reverse means included in said interrogating means oper- .ated responsive to the operation of said evaluating means to connect said supply means in a second interrogating position with said .positive terminal connected to said first conductor said negative terminal connected to said second conductor -thereby changing the markings of said conductors and operating said evaluating means .to evaluate the marked conductors to determine the operated digit signalling means.

2. A signalling system Ias set forth in claim 1 wherein loop relay means are provided in said interrogating means for -connecting said interrogating means in a third interrogating condition with voltage removed from one of said conductors, said loop relay means operated responsive to the operation of predetermined ones of said digit signalling means.

3. A signalling arrangement as set forth in claim 1 wherein the said digit signalling means includes means for marking only one of said conductors when said conductor has a voltage of a given polarity thereon.

4. A signalling arrangement as set for-th in claim 1 wherein said evaluating means includes polarized relay means associated with said line conductors.

5. A signalling system as set forth in claim 1 wherein said means for evaluating includes a pair of polarized detector means connected to each of said conductors and wherein said markings from each of said digit signalling means operates a unique combination of said detector means.

References Cited by the Examiner UNITED STATES PATENTS 2,278,410 4/ 1942 Bascom et al 179-42 2,306,087 12/ 1942 Toffel 179-90 2,364,771 `12/ 1944 Bascom et al. 340-352 2,400,148 5/ 1946 Hubbard 340-349 2,495,725 1/ 1950 Horwitz 179-16 2,712,571 7/ 1955 Hjerstrand 340-352 ROBERT H. ROSE, Primary Examiner.

WALTER L. LYNDE, Examiner.

20 H. BOOHER, H. ZELLER, Assistant Examiners. 

1. A SIGNALLING ARRANGEMENT FOR A PUSHBUTTON TELEPHONE DIALLING SYSTEM COMPRISING CENTRAL OFFICE EQUIPMENT SERVING A PLURALITY OF TWO CONDUCTOR LINES, EACH HAVING A SUBSCRIBER STATION THEREON, VOLTAGE SUPPLY MEANS IN SAID CENTRAL OFFICE HAVING A POSITIVE VOLTAGE TERMINAL FOR SUPPLYING A POSITIVE VOLTAGE WITH RESPECT TO GROUND AND A NEGATIVE VOLTAGE TERMINAL FOR SUPPLYING NEGATIVE VOLTAGE WITH RESPECT TO GROUND, INTERROGATING MEANS IN SAID CENTRAL OFFICE NORMALLY CONNECTING SAID SUPPLY MEANS IN A FIRST INTERROGATING POSITION WITH SAID NEGATIVE TERMINAL CONNECTED TO THE FIRST OF SAID TWO CONDUCTORS AND SAID POSITIVE TERMINAL CONNECTED TO THE SECOND OF SAID TWO CONDUCTORS, A PLURALITY OF DIGIT SIGNALLING MEANS IN EACH OF SAID STATIONS SELECTIVELY OPERABLE TO MARK SAID CONDUCTORS IN DESIRED PERMUTATIONS AND COMBINATIONS, SAID PERMUTATIONS AND COMBINATION COMPRISING INDIVIDUALLY GROUNDING THE FIRST CONDUCTOR, INDIVIDUALLY GROUNDING THE SECOND CONDUCTOR, GROUNDING BOTH CONDUCTORS SIMULTANEOUSLY, GROUNDING NEITHER OF THE CONDUCTORS AND CONNECTING BOTH CONDUCTORS TOGETHER, EVACULATING MEANS IN SAID CENTRAL OFFICE MOUNTED BETWEEN EACH OF THE CONDUCTORS AND GROUND AND OPERATED RESPONSIVE TO SAID MARKINGS, AND REVERSE MEANS INCLUDED IN SAID INTERROGATING MEANS OPERATED RESPONSIVE TO THE OPERATION OF SAID EVALUATING MEANS TO CONNECT SAID SUPPLY MEANS IN A SECOND INTERROGATING POSITION WITH SAID POSITIVE TERMINAL CONNECTED TO SAID FIRST CONDUCTOR SAID NEGATIVE TERMINAL CONNECTED TO SAID SECOND CONDUCTOR THEREBY CHANGING THE MARKINGS OF SAID CONDUCTORS AND OPERATING SAID EVALUATING MEANS TO EVALUATE THE MARKED CONDUCTORS TO DETERMINE THE OPERATED DIGIT SIGNALLING MEANS. 